1. Field of the Invention
The invention relates generally to switching power supplies. More particularly, the invention relates to a switching power supply having a high efficiency starting circuit.
2. Description of Related Technology
Generally speaking, a switching power supply (SPS) provides a cost effective and energy efficient device for converting energy from a single direct current (DC) supply voltage into one or more DC output voltages that have a greater or lesser magnitude than the supply voltage. Traditionally, a SPS has an integrated control circuit that modulates the duty cycle of a transistor switch, which controls the flow of energy into the primary of a transformer to produce one or more desired output voltages that are derived from the secondary of the transformer. As is well known, the energy (i.e., the time integral of power) supplied to the primary of the transformer minus efficiency losses equals the energy transferred to the secondary of the transformer. Thus, if more energy (i.e., voltage and/or current) is needed by the secondary, then the control circuit increases the duty cycle of the transistor switch to provide more energy to the primary. Conversely, if less energy is needed by the secondary, then the control circuit decreases the duty cycle of the transistor switch.
FIG. 1 is an exemplary schematic diagram of a conventional SPS, which includes a direct current (DC) voltage supply block 10, a voltage output block 20, a feedback block 30, and a switching control circuit 40. The DC voltage supply block 10 includes a bridge rectifier 1 and a filter capacitor C1. The bridge rectifier 1 rectifies alternating current (AC) line voltage to produce current pulses which are substantially smoothed to a DC supply voltage Vcc by the filter capacitor C1. For example, if the AC line voltage is 110 volts AC, then the smoothed DC supply voltage across capacitor C1 may be approximately 155 volts DC.
The output voltage block 20 includes a switching transformer 22 having a primary winding L1 and secondary windings L2 and L3 and switching rectifier diodes D5 and D6 that receive current pulses from the respective secondary windings L2 and L2 to provide rectified current pulses to respective filter capacitors C2 and C3. The filter capacitors C2 and C3 smooth the rectified current pulses to substantially DC voltages.
The feedback block 30 includes a voltage feedback amplifier 3 and a photo-coupler 4. The feedback amplifier 3 detects the DC voltage across the filter capacitor C2 and provides a proportional current to the photo-coupler 4.
The switching control circuit 40 includes a pulse width modulated (PWM) signal generator 5, a switching transistor M1, and a feedback capacitor C4. The switching transistor M1 is connected to the primary L1 of the transformer 22 and is switched on and off by the PWM signal generator 5 at a duty cycle that is based on the magnitude of a feedback voltage VFB on the feedback capacitor C4.
Initially, when AC line voltage is first provided to the bridge rectifier 1, the supply voltage Vcc applied to the PWM signal generator 5 is substantially near zero volts DC and the PWM signal generator 5 is off. Additionally, because the PWM signal generator 5 is off, the switching transistor M1 is off, energy is not being provided to the primary winding L1, and the output voltages across capacitors C2 and C3 are substantially near zero volts DC.
As is generally known, the PWM signal generator 5 is typically fabricated using conventional integrated circuit technologies and requires a relatively low DC supply voltage, which may be, for example, between 4 volts DC and 12 volts DC. Typically, the low supply voltage required by the PWM signal generator 5 is derived from the output voltage block 20. Thus, as shown in FIG. 1, the supply voltage Vcc for the PWM signal generator 5 is connected to the voltage across capacitor C3. Additionally, because the voltage across capacitor C3 is initially substantially near zero volts DC, a start up resistor R is connected between capacitors C1 and C3. The start up resistor R provides an initial charging current to capacitor C3 that causes the voltage across C3 to increase. Once the voltage on C3 reaches a level sufficient to cause the PWM signal generator 5 to begin functioning, the voltage across C3 is regulated by the operation of the PWM signal generator 5 and the current flowing through the start up resistor R no longer increases the voltage on C3.
Although the start up resistor R is needed to the start the operation of the PWM signal generator 5, the start up resistor R becomes a significant source of energy inefficiency once the PWM signal generator 5 is operational. More specifically, a large voltage differential exists across the start up resistor R because the difference between the output voltage of the supply block 10 is substantially greater than the low voltage supply Vcc for the PWM signal generator 5. For example, the output voltage of the supply block 10 may be 155 volts DC while the low voltage supply Vcc is 5 volts DC. This large voltage drop during continuous operation of the SPS results in a significant source of energy inefficiency.